This invention relates to the means of controlling the flow rate through the dynamic compressor having rotating vanes.
Control systems of such compressors are designed for changing their performance to fit the requirements of the user's process.
On the other hand, in order to protect the compressor from approaching a zone of instable operation, a control system must provide for limiting a possible safe range of changing the above performance. The known conventional compressor control systems are supposed to solve the last named problem by using two or more independent control loops operating in parallel.
One of these (this loop henceforth will be called "process control loop") controls the process parameter, for instance, mass flow rate, by changing the performance of the installation. Another loop limits the range of changing the above performance in an indirect way, using blowing-off or recycling of a compressed gas in order to provide a required change of an equivalent resistance of a delivery network (the load characteristic).
If, for example, while maintaining constant mass flow rate, the load increases, then the discharge pressure can reach a permissible limit. At this moment the process control loop and the protective control loop begin to operate simultaneously. During this transient period, the process control loop continues to change the performance and this can interfere with protective systems designed to keep the compressor from approaching the surge zone, especially in cases when the protective control loop controlling a relief valve includes one or more elements having nonlinearities like hysteresis or dead zones.
This disadvantage may be eliminated by using a noninteracting control and protective system of a dynamic compressor with the rotating vanes.
According to "Process Measurement & Control Terminology," SAMA Standard, PMC 20-1970, page 11, a noninteracting control system is a "multi-element control system designed to avoid disturbances to other controlled variables due to the process input adjustment which are made for the purpose of controlling a particular process variable."
Introducing a compressor control system of the above mentioned type is the main goal of this invention.